Right click on the white boost press box , go to the right of the page you opened and look at the smooth depth (points) and slide the scale up to about 9 or 10 and look at your original graph again and see what has changed .

I think what your are seeing is pressure fluctuations or pulses from the rotors. I think it will go away if you restripe the blower. DQUES is correct that you can use smoothing to hide it but it is still there.

Staff Member

I would talk to Racepak and see what the "normal" smoothing is. After working with some other non-racepak Data Loggers, you find that Racepak has it pretty scienced out on what needs to be smoothed and by how much. When you get an Autometer for instance, you pretty much have to put all the smoothing in yourself, otherwise you get raw data, which will have fluctuations like you show.

If your smoothing is normal, it may be the pressure transducer. I just have a hard time justifying that as real data. With an anomaly on something as central to the performance of the car as the supercharger, I have a hard time believing you wouldn't see corresponding fluctuations in Engine RPM and EGT. I have never seen a blower 'pulse' like described, but I have recently had a problem with the air throttle on a Top Dragster with it partially closing in high gear, resulting in a 3-4 lb drop in boost. An erratic RPM trace corresponded with it.

Even if there was corresponding fluctuations in EGT's, how would that not effect engine RPM in the slightest way? I will concede there are very, very minor 'bumps' in the engine trace, but I just have a hard time with it.

One more sign of evidence it may be a bad transducer or vnet cable picking up RFI is the big dips in the trace towards the finish line.

If the data is real, the only rational explanation, and I don't have a bunch of experience with roots, is the blower is 'cavitating.' Could be from bad strips? That said, if you have ever seen a fuel pump go into cavitation, it usually completely loses pressure. Never heard of a blower doing that, but again, if it was that bad I have a hard time believing you wouldn't see it in performance or engine rpm.

I don't think it's possible for a positive displacement pump with air as a working fluid to cavitate.

Cavitation as I understand it is simply the local vaporization of the working fluid (liquid to gas phase change) typically due to a restriction on the suction side.

I assume you could get reversion due to really loose clearances, worn out or no strips, and an inlet restriction causing low pressure above the rotors. Ive never seen any discussion on that topic, though.